1. Field of the Invention
The present invention relates to a waveguide-microstrip line converter for transmitting signals transmitted through a waveguide packed with a dielectric to a microstrip line without signal transmission loss.
2. Description of the Prior Art
The dominant mode of a general rectangular waveguide is TE mode while the mode of a microstrip line is TEM mode. Therefore, the rectangular waveguide and the microstrip line need to be interconnected by a converter with a sufficiently small signal transmission loss.
In FIG. 5, illustrating a conventional waveguide-microstrip line converter, there are shown a short-circuit waveguide 21, a probe 22, a coaxial center conductor 23, a coaxial dielectric body 24, a microstrip line 25, a connecting member 26, a mount 27 and a MIC substrate 28. The signal transmitted through the short-circuit waveguide 21 is received by the probe 22. The probe 22 is attached to the free end of the center conductor 23. The center conductor 23 extends through a hole formed in the wall of the short-circuit waveguide 21 and the other end of the center conductor 23 is connected to the microstrip line 25 with a connecting member 26. A dielectric element 24 is provided on the center conductor 23 to insulate the center conductor 23 from the short-circuit waveguide 21 and to fix the center conductor 23 to the short-circuit waveguide 21. Thus the signal transmitted through the short-circuit waveguide 21 is received by the probe 22 and the waveguide mode is converted properly into the microstrip line mode in transmitting the signal through the center conductor 23 and the connecting member 26 to the microstrip line 25. The diameter and length of the probe 22, the length of a portion including the center conductor 23 and the probe 22 projecting inside the short-circuit waveguide 21, and the location of the probe 22, namely, the distance between the end of the short-circuit waveguide 21 and the center of the probe 22 and the distance between the side wall of the short-circuit waveguide 21 and the center of the probe 22, are decided properly for best impedance matching. The microstrip line 25 is formed by printing a conductor on the MIC substrate 28 attached to the integral mount 27 of the short-circuit waveguide 21.
Since the probe 22 is inserted through the hole formed in the wall of the short-circuit waveguide 21 into the short-circuit waveguide 21 so as to project from the inner surface of the short-circuit waveguide 21, it is difficult to adjust the length of projection of the probe 22 inside the short-circuit waveguide 21 properly in attaching the probe 22 to the short-circuit waveguide 21, and hence the length is often different from a predetermined correct length. Since the probe 22 is fixed at a portion corresponding to the hole formed in the short-circuit waveguide 21, it is difficult to attach the probe 22 to the short-circuit waveguide 21 so as to extend at right angles to the wall surface of the short-circuit waveguide 21. Consequently, the probe 22 is liable to be located inaccurately relative to the end and side wall of the short-circuit waveguide 21. Furthermore, in soldering the connecting member 26 to the center conductor 23 and the microstrip line 25, it is difficult to solder the connecting member 26 to the center conductor 23 and the microstrip line 25 at correct position with an appropriate amount of solder. All these difficulties entail a problem that impedance matching is deteriorated in converting the waveguide mode into the microstrip line mode. Since the center conductor 23 and the microstrip line 25 are interconnected by the connecting member 26, the electromagnetic radiation from the connecting member 26 causes signal transmission loss and large signal transmission loss makes broad-band transmission impossible. Moreover, since the probe 22 is secured at a portion corresponding to the hole formed in the wall of the short-circuit waveguide 21 to the short-circuit waveguide 21, the probe 22 is unstable when subjected to vibrations, and hence the location of the probe 22 is liable to change with time.